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Old 9th November 2009, 12:56 PM   #1
Zekk is offline Zekk  Singapore
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Default 1.5VDC Heater Supply For #26!?

I am a newbie ... don't laugh ya !

OK ... wish to build a #26 linestage but having problem getting the 1.5VDC required for the #26 tube!

Built Chris Brady's design : 5VAC~Schottky bridge~10K uf~0.47R~10K uf~LM1086~1.2R . I got 4.7VDC !? From here, how do I acheives the 1.5VDC@1A needed for the Type 26 tube?

Thanks for looking & any help forecoming,

Zekk
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Old 9th November 2009, 01:03 PM   #2
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Quote:
Originally Posted by Zekk View Post
I am a newbie ... don't laugh ya !

OK ... wish to build a #26 linestage but having problem getting the 1.5VDC required for the #26 tube!

Built Chris Brady's design : 5VAC~Schottky bridge~10K uf~0.47R~10K uf~LM1086~1.2R . I got 4.7VDC !? From here, how do I acheives the 1.5VDC@1A needed for the Type 26 tube?

Thanks for looking & any help forecoming,

Zekk
Hey.

Don't know about this design, but you mention the 1.2R resistor, and therefore I think the LM1086 is configured as a CCS (the 1.2R between adjust and output, and the tube filament fed from 'adj'). This means that the regulator will pass only 1A, which multiplied by the Resistance of the 26's cathode will develop 1,5V. Prior to testing that, just post the schematic!

Erik
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Old 9th November 2009, 01:13 PM   #3
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In principle that should work, with LM1086 as current source. that means 1.2R between adj and out pins. Be careful to take the output from the adjust pin!! This may be where you are going wrong.

To make sure you have enough headroom, just try taking the capacitor straight to the LM1086 input. you need about 4.5v going into the LM1086. I use a 6v secondary. But I suspect you've wired the LM1086 wrong, as above.

andy
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Old 9th November 2009, 01:28 PM   #4
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And don't forget to measure the output voltage with a tube or equivalent resistor connected at output.
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Old 9th November 2009, 01:30 PM   #5
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by Zekk View Post
Built Chris Brady's design : 5VAC~Schottky bridge~10K uf~0.47R~10K uf~LM1086~1.2R . I got 4.7VDC !? From here, how do I acheives the 1.5VDC@1A needed for the Type 26 tube?
If that is a CCS filament supply you cannot measure the voltage at its output with no load. If you want to measure voltage, you need to put a stable load there (i.e. tube filament, but wait with your measurement until it is heated up to its working temperature as the readings will change along with its resistance, which changes with temperature). If you're worried about your tube you can use a 1.5R 2+W (!) resistor instead.

LM1086 is a low-dropout regulator so it only needs ~1.5V voltage above the desired output, which would explain your high voltage measurement (4.7V) with open terminals.

By the way, make sure your LM1086 has some sort of cooling (= a suitable heatsink) as it is going to dissipate approximately 5 watts during operation.
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Old 10th November 2009, 12:33 AM   #6
Zekk is offline Zekk  Singapore
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Default Thank You!

First and foremost, many thanks to all the kind soul out there who've given me the help to build this unique linestage.

Loaded with a 1.5R resistor as adviced and there it is, a 1.5VDC reading!

Cheers!

Zekk
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